Lignin containing fibers such as unbleached chemical and semi-chemical pulps are frequently used for products in which inferior optical and inferior strength characteristics are acceptable. In general, however, the fiber bonding lignin is separated from the cellulose fibers by delignification, that is, the lignin removal resulting in fiber delamination.
Commercial chemical delignification processes for the selected removal of lignin include acid sulfite pulping, soda pulping, alkaline or so-called kraft pulping which has been adopted world wide, the alkaline-sulfite-process basically attempting to combine the technology of the latter two processes, the neutral sulfite semi-chemical (NSSC) pulping process, and finally, more selective methods for removing residual lignin such as during known bleaching processes including hypochlorite solutions, chlorine dioxide with or without the addition of chlorine, and bleaching technologies based on oxygen and ozone.
After the pulping the digested chips maintain their chip form mainly due to the presence of cohesion forces, although the chips have been cooked to or beyond the defibration point, whereby the lignin has been dissolved from between the individual fibers so that chemical bonds between adjacent fibers no longer exist. The fibers remain parallel to each other due to the presence of mechanical bonds, which do not normally break when the digester is discharged. The pulp which is discharged in form of chips from a continuous digester creates severe problems in the later treatments and, in diffusers whose washing efficiency drops drastically if the pulp in chip form is introduced in the diffuser. The same problem, but less severe, is encountered with other types of digesters and washers.
One way to solve the problem is to dilute the digested pulp in the high-heat zone of the continuous digester to such a low consistency that the pressure difference in the discharge of the digester is sufficient for fiberizing the soft chips. This, however, increases the liquid consumption and thus causes higher environmental problems relating to waste waters and chemical recovery.
U.S. Pat. No. 4,002,528 discloses an apparatus for refining digested pulp in which two or more refiners are positioned in series in the blow line of a continuous digester. The washed, digested material is fed directly into the refiners in which undigested knots and shives are ground into small particles with the objective to render unnecessary the conventional screening operations. The major disadvantage of this procedure is that knots, even after they have been refined into small particles still have a kappa number of about 100 and are unbleachable and are thus diminishing the quality of the pulp. Accordingly, refiners cannot be used in the production of pulp having a low kappa number, i.e. high quality pulp. An additional disadvantage results in the fact that the action of one or more rotary grinding disks of the refiner tends to break up and further divide the individual fibers which is mostly undesirable.
U.S. Pat. No. 4,971,658 discloses the extraction of lignin by washing from alkaline pulps at a consistency of 3-30% after pressurized pre-washing at elevated temperature. It is further disclosed that the extraction of lignin is increased and the kappa number of pulp further decreased by fiberizing the digested material after pre-washing and by carrying out the extraction at elevated temperature.
U.S. Pat. No. 4,737,274 discloses a tramp material separator for the effective separation of unwanted contaminants such as metal, gravel, stones, nuts, knots, and inordinately large wood fiber bundles, or the like. The pulp suspension is fluidized by a specially shaped rotating disk comprising teeth-like structures, or pegs, for moving the contaminants outwardly through a separated particles outlet into a storage container. The pulp freed from the contaminant particles passes through an annular opening into an outlet chamber from which it is pumped out from the separator.
It is also known to arrange a so called blow unit in the blow line between the digester and the diffuser. The blow unit is intended to defiberize high lignin content or high yield pulps, but this has never been achieved. Nowadays in practice all the blow units have been removed as inoperable as they only consume energy (about 20 to 30 kW) without accomplishing their expected objective. The volume of the blow unit is about 620 liters, whereby the maximum energy fed in the blow unit is approximately 30/620=0.048 kW/l.